In mobile communication networks, there is a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the mobile communication network is deployed.
One performance and capacity parameter of the communication network relates to utilization of orthogonal time/frequency resources (inter alia Orthogonal Frequency Division Multiplexing (OFDM) in the downlink (DL) and Single Carrier Frequency Division Multiple Access (SCFDMA) in the uplink (UL)). Certain utilization of the orthogonal time/frequency resources may allow scheduling multiple user equipment (UE), at the same time over the operating bandwidth without creating any intra-cell interference (particularly when assuming that spatial multiplexing or multi-user multiple-input and multiple-output (MU-MIMO) mechanisms are not used).
In order to schedule UEs, whether in DL or in UL, the UEs should be informed on which frequency resources they are expected to transmit/receive data, which Modulation and Coding Scheme (MCS) to use, etc.
In mobile communication networks based on the Long Term Evolution (LTE) telecommunications standard this may be accomplished by means of the physical downlink control channel (PDCCH). In LTE the PDCCH is typically broadcasted every millisecond over the first one, two or three OFDM symbols (out of the 14 OFDM symbols transmitted every millisecond, assuming a normal cyclic prefix). The PDCCH assignments to the UE are encapsulated into control channel elements (CCE) whose purpose is mainly to simplify the search for the UE on the PDCCH.
The PDCCH is transmitted in the control region, typically the first one, two or three symbols of a subframe, using 1, 2, 4, or 8 CCEs. The number of CCEs selected for a PDCCH depends on the Downlink Control Information (DCI) format and coding rate, i.e. the link adaptation for PDCCH. Each CCE consists of 36 Resource Elements (RE). The size of CCE-space is between 1 and 88 CCEs depending on downlink system bandwidth, number of antenna ports, Control Format Indicator (CFI), physical hybrid automatic repeat request indicator channel (PHICH) resources size, cyclic prefix size and Time Division Duplex/Frequency Division Duplex (TDD/FDD) configuration.
PDCCH is a resource shared by both UL and DL UEs. As a consequence thereof the PDCCH needs to be large enough (i.e. occupy enough bandwidth and/or other resources) to schedule a plurality of UEs in every Transmission Time Interval (III) in case of a plurality of simultaneously active UEs. At the same time, a larger PDCCH results in a smaller physical downlink shared channel (PDSCH) as they both share the same resources, which in turn thus implies fewer resources to be available for transmission of the actual payload data in the DL. This may thus further imply losses in both peak throughput and cell capacity to be caused.
One concept in particular that requires efficient usage of PDCCH is the “shared cell” concept. In all simplicity, the shared cell concept refers to configuring two adjacent cells with the same Physical Cell Identity (PCI). One of the main advantages of this concept is avoiding handover between the adjacent cells sharing the same PCI as these cells will appear as one single cell for the UE.
In shared cell deployments, the PDCCH can easily become a bottleneck. A proper handling of PDCCH power may provide a high capacity for the PDCCH (i.e. have enough UL grants for scheduling uplink users and DL assignments for scheduling downlink users) without compromising the performance for UEs at the cell-edge, especially as these UEs are expected to reap the benefits of a shared cell deployment. Conventional solutions, especially for legacy systems (i.e. pre-Release 10 and 11 of LTE) have one of two options. A first option involves reusing the PDCCH in cells with the same PCI. This has the advantage of potentially high capacity but it has also the main limitation of severely degrade the PDCCH, especially for UEs at the cell-edges since at the cell-edges UEs may receive DL signalling from an adjacent cell. A second option involves using a common (or shared) PDCCH in cells with the same PCI. This has the advantage of protecting UEs at cell-edges at the expense of a limited capacity since the PDCCH are shared over more than one cell. This limitation becomes more significant for increasing number of cells with the same PCI.
Hence, there is still a need for an improved handling of PDCCH power in shared cell scenarios.